This application claims benefit of Japanese Application No. Hei 11-345678 filed in Japan on Dec. 6, 1999, the contents of which are incorporated by this reference.
The present invention relates to light collecting optical systems and, more particularly, to a laser beam collecting optical system using a wavefront converting element capable of arbitrarily changing the wavefront shape.
It has heretofore been necessary in order to obtain a three-dimensional image of a specimen in a laser scanning microscope, for example, to capture optical images of successive planes inside the specimen by mechanically moving either the specimen or the objective in the direction of the optical axis. With this method, however, it is difficult to realize accurate scanning in the optical axis direction in terms of position control errors, reproducibility, etc. because the specimen or the objective is mechanically moved. In a case where the specimen is scanned, high-speed scanning cannot be performed when the specimen is large in size. Furthermore, in a case where the objective is immersed directly in a biological specimen or a culture solution to observe the high-speed movement of living organisms, if the objective is scanned, adverse effects such as vibrations are exerted upon the specimen to be observed.
As an optical apparatus provided with an optical element capable of changing power to solve the above-described problems, Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-101942 discloses an adaptive optical apparatus for use in a microscope. In this prior art, a wavefront modulator is inserted in either or both of an observation optical path and an illumination optical path. The focal length of the optical system is changed by using the wavefront modulator, and aberration due to the change of the focal length is also absorbed by controlling the amount of wavefront modulation as a function of the pupil radius. By doing so, it is possible not only to form and move a focal point in the object space without changing the distance between the objective and the specimen but also to correct aberration.
If the focal point is moved in the object space without changing the distance between the objective and the specimen, spherical aberration occurs owing to the movement of the focal point. The prior art does not specifically state how the wavefront is to be converted by the wavefront converting element to correct the aberration.
The present invention was made to solve the above-described problems associated with the prior art. Objects of the present invention are as follows. A first object of the present invention is to scan the position where a light beam is collected along the optical axis direction in a light collecting optical system by using a wavefront converting element capable of optically changing power without using a mechanical device, and to cancel aberration occurring during the scanning by using the wavefront converting element to thereby minimize the degradation of light collecting performance due to the scanning along the optical axis direction. A second object of the present invention is to express the properties of a wavefront shape produced by the wavefront converting element. A third object of the present invention is to clarify the mechanism as to how the wavefront shape converted by the wavefront converting element changes according to the position where light is collected in the optical axis direction of the objective.
To attain the above-described objects, the present invention provides a light collecting optical system having a light source and an objective for collecting light from the light source. The light collecting optical system further has a wavefront converting element placed at or near a position conjugate to the pupil position of the objective.
The wavefront converting element changes a wavefront shape formed in an area covering at least 90% of a light beam at or near the pupil position according to the following conditions 1 to 4.
When the position where the light from the light source is collected by the objective is away from the objective, the wavefront converting element changes the wavefront shape according to the following conditions 1 and 2.
Condition 1: the amount of change of the wavefront shape decreases monotonously from the center toward the periphery of the pupil.
Condition 2: the wavefront shape is convex toward the objective.
When the position where the light from the light source is collected by the objective is closer to the objective, the wavefront converting element changes the wavefront shape according to the following conditions 3 and 4.
Condition 3: the amount of change of the wavefront shape increases monotonously from the center toward the periphery of the pupil.
Condition 4: the wavefront shape is concave toward the objective.
Thus, even when the position where light is collected is changed along the optical axis direction in such a manner that the relative distance between the objective and the specimen does not change, spherical aberration occurring owing to the change of the position where light is collected can be minimized.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.